The present invention relates to offshore structures and more particularly relates to a method and apparatus of providing a "guyed tower" offshore structure with conductors which are capable of staying within acceptable curvature limits when the longitudinal axis of said guyed tower rotates from the vertical.
As the production of hydrocarbons from marine deposits moves into deeper and more hostile waters, the problems associated with providing the offshore structures required for drilling and producing these hydrocarbons obviously increase. Recently, various offshore structures have been proposed as possible alternatives for the commonly-known and widely used fixed platform; especially in those offshore regions where the use of a fixed platform is technically and/or economically infeasible, e.g. extreme water depths, hostile surface conditions, etc.
One such proposed structure is one which is commonly referred to as a "guyed tower" platform which is basically a trussed structure having substantially uniform cross-sectional dimensions throughout its length. Although other geometric configurations may be used, the main truss of a typical guyed tower normally has four equally spaced legs connected together primarily with X-bracing members. The main truss of the tower rests on the marine bottom and extends upward to a point above the surface. A drilling/production deck is affixed onto the top of the main truss above the normal wave heights of the water body. The main truss is maintained in a substantially vertical position in the water by means of a plurality of guylines which are positioned symmetrically about the truss. For a more complete description of a typical guyed tower offshore structure, reference is made to a paper "A New Deepwater Offshore Platform--The Guyed Tower", L. D. Finn. Paper No. 0TC 2688, presented at the Offshore Technology Conference, Houston, Texas, May 3-6, 1976.
In using a guyed tower such as described above to drill and/or produce underwater formations, conductor conduits will normally be run through guides on the tower after the tower has been installed on location. As understood in the art, these conductors extend from the surface and are driven to refusal or otherwise penetrated to a desired depth in the marine bottom. A well is then drilled and/or completed through each of these conductors by techniques, e.g. directional drilling, well known by those skilled in the art.
Due to the inherent compliancy of the guyed tower, the longitudinal axis of the guyed tower will tilt or rotate from vertical whenever the tower experiences certain surface conditions, e.g. high wave and wind action. This tilting or rotation of the tower about its "center of rotation" (to be defined below) can seriously affect the conductors near the marine bottom if the rotation exceeds certain, relatively small limits and the center of rotation is below the depth of the competent soils of the marine bottom. For example, analysis of a typical guyed tower reveals that tower rotation of one-and-a-half degrees from the vertical will cause excessive well curvatures (i.e. in access of the accepted limit of 6 degree curvature per 100 feet) and conductor stresses in conventionally supported wall conductors if the center of rotation of the tower is located at or below the depths of the competent soils of the marine bottom. "Competent soils" as used here is defined as those soils which are sufficiently competent to resist any substantial lateral movement of the conductors when the longitudinal axis of the tower rotates from vertical. If the center of rotation of the guyed tower is near or below the depth of the competent soils, rotation of the tower will generate an undesirable S-shaped deflection in a conventionally installed well conductor in that interval of the conductor which lies between the lowermost conductor guide on the tower and the competent soil levels. The greater the depth of the center of rotation, the greater the lateral movement of the tower at the marine bottom will be, thereby causing higher conductor curvatures and stresses.
Of course, in some instances, an extremely heavy and rigid conduit, e.g. extremely thick-walled casing, can be used to form the entire length of the conductor to provide additional strength in resisting bending thereby reducing the conductor curvature and stress problems developed by the rotation of the tower. However, in addition to the obvious expense and handling problems involved with installing the required lengths of such heavy conduit, the weight of the conductor, itself, becomes a problem in that the vertical weight load on the conductor may cause failure due to buckling or the like. Therefore, this does not appear to be the solution needed.